The present invention is related to battery chargers. More specifically, the present invention is related to a battery charger that is capable of detecting the presence or absence of a battery connected across the battery charging terminals.
Diesel firepump controllers and other systems that contain battery charging circuits often require a means to determine if a battery is connected to the charging circuit. Prior art systems typically monitor the output current of the charging circuit to verify the presence of a battery. Unfortunately, simply monitoring the output current of the charging circuit is not sufficient under all circumstances to verify connection of the battery across the charging terminals.
By way of example, additional circuit loading on the battery charger can mask the loss of low levels of float charge current, and prevent the detection of a disconnected battery. Also, external charging sources, such as engine-driven alternators, can temporarily raise the battery terminal voltage above float charge levels which will terminate the charging circuit""s float charge current. Simple current monitoring schemes will falsely report battery disconnection in this case.
An example of a prior art battery detector is disclosed in U.S. Pat. No. 5,821,730 to Drapkin. The battery detection circuit uses a sensing resistor, in combination with a transformer, which has a dual secondary winding to sense current to the battery. When no load is present, there is little or no current through the sensing resistor, and a diode connected to the secondary winding is reverse biased. When a battery is present, current through the sensing resistor causes the diode to be forward biased, which in turn causes a step up in voltage at a node which turns on a transistor, indicating that a battery is present. Battery detecting techniques such as the battery detector disclosed in this patent fail to detect a false load which allows current to flow through the sensing resistor, resulting in a false battery detection.
Another example of a prior art battery detector is disclosed in U.S. Pat. No. 5,825,100 to Kim. This battery detector is an example of an electro-mechanical battery detector. When a battery is present, conduction surfaces are caused to contact connection points. The closed circuit between the connection points indicates that a battery is present. Unfortunately, this battery detection circuit will not detect whether the object inserted is actually a battery or not. Any physical object which causes the conduction surfaces to contact the connection points will be detected as a battery.
Accordingly, it is an object of the present invention to correctly detect the connection or disconnection of a battery from a charging circuit. A battery detector according to an embodiment of the present invention accurately detects the presence of a battery, even when external charging sources such as an engine alternator temporarily stop the charging circuit""s current. A battery detector according to an embodiment of the present invention also accurately detects a disconnected battery, even in the presence of an unexpected circuit load that may draw current from the charging circuit.
These and other objects of the present invention are achieved by connecting a relatively low value inductor in series between the battery and the charging circuit. The series inductor provides a low resistance path for battery charging current when the battery is present. A relay and a capacitor are connected in parallel to each other and in series with a switching transistor. The relay, capacitor, and transistor combination is connected in parallel across the battery connector terminals. The transistor is activated periodically by a series of pulses. When a battery is present, current from the battery flows through the low resistance path to the capacitor, and energizes the capacitor and the relay when the transistor is turned on (during each pulse). If a battery is not present, the series inductor provides a high impedance path from the charging circuit to the capacitor and relay, preventing current from charging the capacitor, and in turn causing the relay to de-energize. A device according to the present invention works correctly, even in the presence of additional loading on the charging circuit or elevated battery voltage.